Recently, an organic EL(electroluminescence) display that uses an organic EL is element emitting light using an organic compound has attracted attentions. Since organic EL elements self-illuminate, provide a fast response, and consume low power, they do not require a backlight. Such organic EL elements are anticipated to be applied to, for example, display units of portable apparatuses.
An organic EL element is formed on a glass substrate and has a structure in which an organic layer is sandwiched between an anode and a cathode. When a current flows in the organic EL element by applying several external voltages, an electron is injected into the organic layer from the cathode and a hole is injected into the organic layer from the anode. When the electron and the hole are injected into the organic layer, an organic molecule is excited. When the excited organic molecule turns back to a ground state as the electron and the hole are recombined, surplus energy is emitted as light.
An organic EL element having high performance may be manufactured when the electron can be efficiently injected from the cathode side into the organic layer by lowering an electron injection barrier while injecting the electron into the organic layer. Accordingly, an electron injection layer formed of a material having a low work function, such as an alkali metal, is generally formed on an interface between the organic layer and the cathode, as disclosed in Non-Patent Documents 1 through 3.
For example, Non-Patent Document 1 discloses that a lithium fluoride (LiF) layer is inserted between a cathode layer formed of aluminum and an electron transport layer, so as to efficiently inject an electron into an organic layer. An alkali metal, such as lithium (Li) or cesium (Cs), has a small work function, and thus may be used to form an electron injection layer.
Non-Patent Document 2 discloses that an alumina (Al2O3) layer is inserted between a cathode layer formed of aluminum and an organic layer (Alq3). Non-Patent Document 3 discloses that a strontium oxide (SrO) layer is inserted between a cathode layer formed of aluminum and an organic layer (Alq3).
Conventional examples of a method of forming an electron injection layer include a vacuum deposition method and a deposition method using an alkali dispenser. Also, the vacuum deposition method is generally used to form a cathode constituting an upper layer of the electron injection layer. Recently, considering the uniformity of deposition of material and an increasing size of a substrate, a sputtering method is is mostly used to form an uniform layer on a large substrate.
In the film-forming process, the cathode may be formed as soon as possible after forming the electron injection layer so as to cover the electron injection layer with the cathode. This is because, as disclosed in Non-Patent Document 1, a material having a low work function is active, and thus may easily react with moisture, nitrogen, oxygen, or the like in a chamber, even when the chamber is in a high vacuum state.
(Non-Patent Document 1) “Enhanced electron injection in organic electroluminescence device using an Al/LiF electrode” 1997 American Institute of Physics, Appl. Phys. Lett. 70(2), 13 Jan. 1997
(Non-Patent Document 2) “Fabrication and electroluminescence of double-layered organic light-emitting diodes with the Al2O3/AI cathode” 1997 American Institute of Physics, Appl. Phys. Lett. 70(10), 10 Mar. 1997
(Non-Patent Document 3) “Thin Film Formation of Strontium Oxide onto Alq and Application to EL Device” (Japanese) Institute of Electronics, Information and Communication Engineers, C-II, Vol. J82-C-II, No. 2, pp. 70-71, February 1999